By way of indication, UWB communication is regulated for frequencies that may range from 3 to 10.6 GHz and the permitted spectral templates are defined, on the one hand, by regulatory constraints (FCC in the United States, ECC in Europe, etc.) and, on the other hand, by constraints imposed by standards (IEEE 802.15.4a, IEEE802.15.6 standards, etc.) or proprietary. These constraints on spectral templates may be grouped under the term “template specification”. The bandwidth is in the range of 1 GHz and the duration of the transmitted pulses is typically of a few nanoseconds. More generally, a UWB signal is defined as a signal whose passband bandwidth at −10 dB of the maximum-power spectral density is higher than 500 MHz, or alternatively whose passband bandwidth is greater than 20% of the center frequency.
By way of example, FIG. 1 shows the general shape of a 5 nanoseconds UWB pulse with a carrier frequency of 4 GHz. FIG. 2 shows the power spectral density of this pulse. This spectrum comprises a main peak at the carrier frequency and a passband bandwidth at −10 dB of around 500 MHz, but it also comprises numerous side lobes whose frequency may be close to the carrier frequency. These peaks run the risk of making the spectrum extend outside the template imposed by the specification, and it is therefore necessary to ensure that they are reduced as much as possible.
The use of fully digital transmitter architectures that allow the spectrum of the transmitted pulse to be effectively controlled in order to reduce the side lobes thereof is theoretically known, but these architectures are not readily compatible with standard integrated-circuit technologies when the aim is to operate with frequencies that are higher than 4 GHz, and even less so at frequencies of 8 GHz. These architectures synthesize a waveform in the time domain in the manner of a digital filter operating at the Nyquist frequency. The value, as well as the precision, of the coefficients of the implemented filter have a direct impact on the level of the side lobes. However, the switching times of digital circuit transistors are too high (at least 16 GHz to operate at an 8 GHz center frequency) in the most commonly used technologies and it would be necessary to use much more expensive technologies to solve the problem.
A transmitter architecture that has been proposed in order to implement UWB pulse transmitters in the band of 3 to 8 GHz is an architecture referred to as an architecture with a switched LC oscillator. It is possible to integrate this architecture in standard technologies (a MOS technology with a transistor channel of 130 nanometers for example).
A generator with a switched LC oscillator is generally constructed as shown in FIG. 3: a pair of intersecting differential branches is mounted as an oscillator; each branch comprises a transistor whose gate is connected to the output of the other branch; the outputs are made at the drain of the transistor of the branch; a load composed of an LC circuit, resonating at the carrier frequency to be transmitted, is connected between the two outputs; the differential pair is supplied with current via a switch that allows or prevents the supply of current to the pair. This switch receives a logic command for turning it on that precisely defines the duration for which the current is applied to the differential pair, hence the duration of the UWB pulse issued at the outputs.
The LC load may be composed of a variable capacitor (for regulating the frequency of oscillation) in parallel with an inductor. The inductor and the capacitor form a resonant circuit at the desired carrier frequency for the UWB pulse. A resistor may be placed in parallel with the inductor and the capacitor in order to regulate the overvoltage coefficient of the resonance. Part (or even all) of the resonance inductor may be composed of the primary of a transformer whose secondary is connected, potentially via a filter, to the transmission antenna that is to receive the UWB pulse.
For communication with frequency modulation, for example, it is possible to act upon the value of the capacitance in order to modulate the frequency from one pulse to the next. The capacitor may then be composed of a varactor diode or equivalent; the capacitance varies depending on a bias voltage applied to the diode and the bias voltage and oscillation frequency vary as a result. It may also be composed of an array of capacitors and selector switches.
For communication with amplitude modulation, it is possible to act upon the value of the supply current of the differential pair. This value defines the peak of the variation envelope of the signal level during the pulse. By modulating the value of the supply current from one pulse to the next, the transmitted pulse is modulated in amplitude and this modulation may be detected in a detector.
However, this type of pulse generator does not allow the phase to be modulated easily as the instant of initiation of the oscillations is random, this type of oscillator initiating naturally due to noise.
The following documents describe UWB pulse generators:    Raphaeli, et al., Ultra Wideband On-Chip Pulse Generator. Patent application U.S. 2010/0177803 A1, July 2010.    Anh Tuan Phan, et al., Energy-Efficient Low-Complexity CMOS Pulse Generator for Multiband UWB Impulse Radio. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, Vol. 55, No. 11, December 2008.    Barras, et al., Low-Power Ultra-Wideband Wavelets Generator With Fast Start-Up Circuit. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 54, No. 5, May 2006.    Pelissier, et al., A 112 Mb/s Full Duplex Remotely-Powered Impulse-UWB RFID Transceiver For Wireless NV-Memory Applications. IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 46, No. 4, April 2011.
The document WO 2011/053243 describes a UWB pulse generator comprising: an oscillator formed by a pair of intersecting differential branches that have two outputs connected to an LC resonant load, the differential branches being supplied with power by a common current that is controlled by a current-switching circuit allowing a current to pass through only for a duration that is equal to a duration of a pulse to be transmitted; two current-injecting branches that are respectively connected to the two outputs; and a phase control circuit that imposes a different injection of current into the two outputs only for a part of the pulse duration.